A method is known from U.S. Pat. No. 5,400,378. The first step in reducing the applied X ray dose during a volumetric transirradiation for recording tomograms is to record shadow images or topograms of each slice of a region to be examined, doing so at an angle of 0° to 90°. An optimum X ray dose and the tube current, corresponding thereto, for the respective slice are then calculated therefrom. Subsequently, each slice is transirradiated with the previously determined optimum X ray dose in order to produce tomograms. The tube current is appropriately modulated over the slices to this end. The proposed method requires a two fold scanning of the patient and thus a high time outlay. Irrespective thereof, the patient is already subjected to the load of a relatively high X ray dose as the topograms are being recorded.
DE 10 2004 043 859 A1 discloses a method for controlling the modulation of the tube current by using a single topogram. In this case, the single topogram is evaluated by using previously stored information relating to the attenuation of the X radiation in a direction orthogonal thereto. The tube current is modulated for each slice in order to attain an optimum X ray dose on the basis of the results attained in this case. The proposed method is certainly less time consuming, because of the production of only a single topogram. However, owing to the approximation methods required to calculate the tube current, inaccuracies can occur that can lead to the application of an X ray dose that is not always optimum.
U.S. Pat. No. 6,393,090 B1 describes a method for operating an X ray computer tomograph in the case of which the slices of the region of the patient that is to be examined are transirradiated at two different angles. A topogram with an item of depth information or a 3D topogram is calculated on the basis of the values measured in the process. In order to carry out the method, the X ray source is held at the first angle, and the patient is then moved along the z-axis and repeatedly transirradiated. Subsequently, the X ray source is rotated and held at the second angle. The patient is again moved along the z-axis and repeatedly transirradiated. The known method is likewise time-consuming. Apart from this, artifacts can be caused during the calculation of the topogram by movements of the organs during the time-offset recording of the slices at different angles.
So-called digital X ray tomosynthesis methods are known from James T. Dobbins III and Devon J. Godfrey “Digital x-ray tomosynthesis: current state of the art and clinical potential”, Phys. Med. Biol. 48 (2003) R65-R106 and from James T. Dobbins III “Chest Radiography, Pt. 3: Chest Tomosynthesis”, http://www.imagingeconomics.com/library/tools/printengine.asp?printArticleID=200505-05. In this case, a region of a body that is to be examined is transirradiated at different angles, a flat detector being used as detector. It is thereby possible, by using suitable algorithms, to produce a tomosynthesis image with a high resolution at a prescribed slice thickness of the body. The method of X ray tomosynthesis can be carried out by using conventional X ray computer tomographs. All that is required to this end is to process the recordings produced at different prescribed angles by using the suitable algorithm to form the tomosynthesis image.
DE 199 25 395 A1 describes a method for operating an X ray computer tomograph. Data obtained during a volumetric transirradiation is extracted in this case in order to produce a topogram. In order to improve the quality of the topogram, the data belonging to a desired projection direction are read out from a number of rows of the detector system and used to reconstruct the topogram. This document makes no statement on setting and/or determining the optimum X ray dose of the radiations used as a function of the respectively transirradiated slices.
EP 0 531 993 B1 describes a method and an apparatus for producing tomograms and topograms by means of an X ray computer tomograph. The patient is transirradiated in a spiral fashion in this case. The measured values obtained during the spiral transirradiation are further processed in order to produce sectional images. At the same time, measured values recorded at a prescribed angle are further processed to form a topogram. Both the sectional image and the topogram are displayed at the same time. A similar method is known from DE 41 03 588 C1. It is proposed in this case that the recording be broken off when a radiologically detected end point is reached.